Hi, I´m the one who started the other "RX100 water color thread" and many of the photos are HDR - with water color applied afterwards (without loosing the original photo of cause).

Is it possible to use full maul controls (incl HDR) when creating water color effect with the RX10?

Yes, you can use full manual controls when you have a picture effect being applied. However, you can't use HDR when using a picture effect. Just tested this out, when I had water color selected, HDR was disabled.